Thin coatings formed of thermoset resins, pigmented and unpigmented, have generally been applied as emulsions; e.g., water dispersed systems or as solutions involving organic solvents. These systems are well known and have been extensively used for many years as the mainstay of the resinous coatings industry.
Solvent based coating systems have a distinct disadvantage since such systems are generally about 60 to 80 percent solvent and only about 20 to 40 percent resin. The organic solvents prevalent for dissolving resins are generally noxious, frequently toxic and are generally considered as air pollutants and health hazards. Water dispersed coatings do not have these disadvantages but thus far they have found little utility as an industrial coating; e.g., as coatings for appliances, automobiles, business equipment, machinery and like, which require, either by reason of service or aesthetics, a high-gloss, easily cleaned surface. Such a surface is not presently available from water based systems. Because of certain federal, state and municipal air pollution standards being imposed and contemplated, the coatings industry is generally without a complying system; i.e., a solvent-free system, to provide thin, high-gloss, easily cleaned surfaces.
Although 100% solids coating systems are available in liquid form; e.g., styrene-unsaturated polyester resin systems, or liquid epoxy, the viscosities of such systems are so high that they cannot be readily sprayed to form thin coatings.
Powder coating systems have, thus far, not offered the full range of properties available in solvent based systems. Powder coating systems generally comprise finely divided particles of thermoplastic or thermosettable resins which can be applied in a fluid bed or by electrostatic spray techniques. The resulting films are generally thick, especially when applied in a fluid bed, and do not have a high gloss, especially when applied by electrostatic techniques. Also, each of these application techniques have certain inherent limitations; e.g., large objects, continuous strips to be coated on one side only, are not readily coated in a fluid bed while some objects such as beverage containers (soft drink and beer cans) cannot be effectively coated on the interior by electrostatic techniques. Also, a coating applied by either technique must subsequently be heated to effect a cure thereof if the coating is thermosettable. Postheating is a further requirement for thicker epoxies or vinyl resins.
The present invention provides a process for applying finely divided thermosettable resin particles to a substrate to form thin, high-gloss coatings which are cured as applied and do not require a subsequent baking. As indicated hereinabove, the process utilizes a plasma spray flame to melt the resin particles as they are propelled towards a substrate.
Although thermoplastic organic resin particles have previously been introduced into a plasma arc flame in order to apply a coating to a substrate, the resulting films have not been ultra thin. Illustrative of such techniques are the disclosures of U.S. Pat. No. 3,179,784 of Johnson, British Pat. No. 1,087,173, and U.S. Pat. No. 3,676,638 of Stand. The emphasis indicated in each of these patents have been directed towards applying PTFE (polytetrafluroethylene) coatings and polyethylene and polypropylene. Although Johnson mentions thermoplastic solid resin particles generally, no specific reference was made to polyvinyl chloride resins and polyvinyl acetate resins, which are predominant thermoplastic powder resins, and which have exhibited unsatisfactory results, as indicated hereinafter, when attempts have been made to apply a coating of same by use of a plasma arc spray technique.
U.S. Pat. No. 3,378,391 of Winzeler also relates to spraying plastic powders by a plasma spray technique. It is indicated therein that an epoxy powder was sprayed which melted and hardened on a substrate. No reference is made therein to cured, thin films of epoxy resins. As indicated hereinafter, a hardened, uncured epoxy film can be applied by liquifying epoxy powder in a plasaspray device.
Thermoplastic particles are similar to metals when processed through a plasma arc flame in that the mechanism of forming a coating involves melting of the particle, directing the particle to the substrate and allowing the substrate and paraticle to cool, whereby the particle again has the same chemical and physical properties. This characteristic of a thermoplastic coating is the same regardless of the method of applying, although in some instances the particle will be applied to the substrate before it is melted; e.g., electrostatic spray.
The mechanism of forming a cured coating from a thermosettable resinous particle is more complex. The particle generally comprises a low molecular weight polymer having reactive sites through which the polymer may cross-link to form a thermoset polymer. The cross-linking rection frequently requires a catalyst to initiate and promote the reaction. The reaction preferably occurs at elevated temperatures, otherwise a catalyzed resinous particle has no storage stability at room temperatures. The cross-linking reaction may be exothermic or endothermic. In either instance, however, it may be necessary to supply heat to the resin to complete the reaction inasmuch as the heat loss from the resin may be greater than the heat generated from the exothermic reaction.